Sound engineering underpins modern landfills, making them safer, more efficient and environmentally sound. Landfills are designed to ensure that the environment is protected including the groundwater, surface water, land and air and that public health is maintained.
After reducing, reusing, recycling, and waste conversion, landfills are used for the final disposition of any material that is managed otherwise. Some points to consider:
- Modern landfills are designed, sited, engineered, operated, regulated, tested and monitored in a safe and environmentally responsible manner.
- Protective landfill liners made of clay and plastic cover the bottom of the landfill preventing landfill liquids, called leachate, from entering the ground or surface waters. This leachate is collected and treated, often at the wastewater treatment plant.
- At the end of each working day, that day’s waste is covered usually with six inches of soil. This soil serves to prevent unwanted animals from digging into the trash, prevents wind-blown litter, and reduces odors. At the end of the landfill’s life, final cover is placed over the entire area. This includes another liner, similar to the one underlying the landfill.
- Modern landfills collect and treat gas that is produced by decomposing organic material in the waste.
- Permitting a modern landfill generally takes between 5 to 7 years, costs millions of dollars and requires rigorous siting, engineering and environmental studies and demonstrations to ensure both state and local environmental and safety concerns are satisfied.
The significant role of new technologies in landfill development and management may surprise some. This is a science-based industry that employs civil and environmental engineers, chemists, soil experts, biologists, geologists and hydrologists to protect today’s environment while developing the sustainable waste management practices of the future.
Anatomy of a Landfill
The bottom liner separates and prevents the buried waste from coming in contact with underlying natural soils and groundwater. In municipal solid waste landfills, the bottom liners are required to consist of both a clay liner and a synthetic one. The synthetic liner is generally constructed using some type of durable, puncture-resistant synthetic plastic HDPE (high density polyethylene) ranging from 30 to 100 mils thick.
This is the area in a landfill that has been constructed and approved for disposal of waste. Cells range in size (depending upon total tons of waste received each day at the landfill) from a few acres to as large as more than 20 acres. The waste is placed in thin layers and compacted by heavy machinery.
Leachate collection system
The bottom of each landfill is typically designed so that the bottom surface of the landfill is sloped to a low point, called a sump. This is where any liquids that drain from the waste — known as leachate — are collected and removed. The leachate collection system typically consists of a series of perforated pipes, gravel packs and a layer of sand or gravel placed in the bottom of the landfill above the liner. Once the leachate is removed from the sump, it is typically pumped to a holding tank or pond, where it is either treated on site or hauled to a wastewater treatment facility.
This is an engineered system designed to control stormwater run-on and runoff. The stormwater is directed through a series of berms or ditches to holding areas known as sedimentation traps or stormwater management ponds. In these ponds the runoff water flow is slowed down or held long enough to allow any suspended soil particles to settle out before the water is discharged off site.
Landfill gas collection system
Over time, bacteria in the landfill break down the organic fraction of the trash. This process produces landfill gas, which is about 50 percent methane and 50 percent carbon dioxide with small amounts of other gases, including trace quantities of some hazardous air pollutants. To control these pollutants, the landfill gas is collected and either flared or beneficially used. The collection system consists of blowers that exert a vacuum through piping connected to a series of wells that are drilled into the waste. This gas, once collected, can be either flared or used for energy.
Waste is required to be covered daily with either six inches of compacted soil or an alternative daily cover. Some examples of alternative daily covers are the application of spray-on cover material, such as foam or a flame-retardant fiber material. Another type of alternative daily cover is large panels of tarpaulin-type material that is laid over the waste at the end of each day and removed the next day before waste is placed. Other areas within the cells that are not to final grade and will not receive placement of additional waste for a period of time may require additional cover. This is known as intermediate cover — generally 12 inches of soil. Covering (or capping) is performed in order to isolate the waste from exposure to the air, pests and to control odors. When a section of the landfill is finished or filled to capacity, it is permanently covered with a combination of a liner, compacted soil and a layer of topsoil and vegetation.
Groundwater monitoring stations
Groundwater monitoring wells are installed around the landfill and tested periodically to verify the landfill performance. Typically a groundwater monitoring system will have a series of wells that are located up-gradient of the landfill disposal area and a series of wells down-gradient. The up-gradient wells test the water quality before it moves under the disposal area in order to get a background analysis of the water. The down-gradient wells then allow testing of the water after it has passed under the disposal area so it can be compared to the quality of the up-gradient wells to make sure there has been no impact or contamination of the groundwater.